The present invention relates to a method for controlling a first arm and a second arm of an articulated robot having two pivotally connected arms.
Recently, by using a microprocessor, the control method for an industrial robot has changed to a digitized system to permit improved operating characteristics and velocity control of a robot arm based on a memorized cam curve whereby velocity instructions may be executed.
More specifically, the cam curve (S curve) having a relationship between time and a displacement amount, as shown in FIG. 1, is digitized and memorized as an S-table beforehand in a memory device and a desired displacement amount is derived one by one based on the cam curve at an operational time and a positional deviation against a present position and is supplied as a velocity instruction to a motor which drives each arm. In order to make the first arm and the second arm start simultaneously and to stop simultaneously, an instruction value derived from the S table is multiplied by a ratio of a displacement pulse amount for a first motor to a displacement pulse amount for a second motor, thereby producing an actual issued desired displacement amount.
As another method for supplying the velocity instruction to each motor, a hybrid control method is known that the cam curve having a relationship between the time and the velocity, as shown in FIGS. 2 and 3, is digitized and memorized in the memory device, and a velocity is derived one by one based on the cam curve at the operational time, and the control for the displacement amount is executed separately based on an integrated value of the issued velocity instruction, or another method using a cam curve having a relationship between the displacement amount and the velocity, as shown in FIGS. 4 and 5, is known.
Essentially, the articulated robot has characteristics such that the moment of inertia changes principally as a function of an angle of the second arm, and a centrifugal force or Coriolis force changes principally as a function of the angle of the second arm and the movement of the first arm and the second arm. Therefore, the load torque presented to a motor or a speed reducer changes largely as a function of the angle of the second arm and the movement of the first and second arms.
However, in the above-mentioned conventional methods, uniform acceleration and deceleration are executed without specifically considering the variation of the load and, therefore, the load torque under the severest conditions should not exceed a tolerance limit; that is, the uniform acceleration and deceleration are executed against an idle operation and the uniform acceleration and the uniform deceleration are substantial obstacles against a shortening of the time to effect a displacement from one point to another.